Referring to the figure above, the transversal AB crosses the two lines and , creating intersections at E and F. Each pair of interior angles are inside the parallel lines and on the same side of the transversal. There are thus two pairs of these angles. In the figure above, click on 'Other angle pair' to show each pair of interior angles in turn.

Remember: interior means inside the parallel lines.

If the transversal cuts across parallel lines (the usual case) then the interior angles are supplementary (add to 180°). So in the figure above, as you move points A or B, the two interior angles shown always add to 180°. Try it and convince yourself this is true. Click on 'Other angle pair' to visit both pairs of interior angles in turn.

If the transversal cuts across lines that are not parallel, the interior angles still add up to a constant angle, but the sum is not 180°.

Drag point P or Q to make the lines non-parallel. As you move A or B, you will see that the interior angles add to a constant, but the sum is not 180°. (The angles are rounded off to the nearest degree for clarity, so bear that in mind if you check this).